1. Field of the Invention
The present invention relates to an diffractive optical element, and to a measuring apparatus using the diffractive optical element.
2. Description of the Related Art
A diffractive optical element that diffracts at least a part of incident light is used in various optical equipments, optical apparatuses, and the like. For example, an optical coordinate measuring apparatus may irradiate light having a predetermined projection pattern on a measuring target, and acquire an image of the measuring target irradiated by the projection pattern of the predetermined light, in order to perform a three-dimensional measurement. In such a coordinate measuring apparatus, a diffractive optical element is used to generate the projection pattern of the predetermined light.
In the coordinate measuring apparatus, there is a demand to project the light over a wide range. For this reason, a diffraction angle of the diffractive optical element becomes large, and there is a tendency for a luminous energy (or quantity of light) of zero order diffracted light, that is, the diffracted light directly transmitted through the diffractive optical element, to become large. When the luminous energy of the zero order diffracted light becomes high compared to the luminous energy of other diffracted lights, blurring and the like may occur in the image obtained by the imaging of the coordinate measuring apparatus in a periphery of the zero order diffracted light and deteriorate the image. Accordingly, it is desirable for the luminous energy of the zero order diffracted light to be low.
An International Publication Number WO2007/105215 A2 and an International Publication Number WO2007/105205 A2 disclose methods of irradiating a speckle pattern generated by the diffractive optical element, as the projection pattern to be irradiated on the measuring target. In addition, an International Publication Number WO2009/093228 A2 discloses a method of reducing the luminous energy of the zero order diffracted light by using two diffractive optical elements.
The method disclosed in the International Publication Number WO2009/093228 A2 inputs light to a first diffractive optical element, and inputs light diffracted by the first diffractive optical element to a second diffractive optical element. Hence, a rectangular diffracted light pattern may be distributed two-dimensionally.
As described above, when the diffractive optical element is used in the coordinate measuring apparatus or the like, there is a demand to distribute the light over a wide range as described above. When two diffractive optical elements are used, a sum of the diffraction angle of the first diffractive optical element and the diffraction angle of the second diffractive optical element becomes the total diffraction angle. Hence, the total diffraction angle becomes wide, and the diffracted light may be distributed over the wide range.
Normally, the diffracted light generated from the diffractive optical element is emitted at a predetermined angle from the diffractive optical element according to a diffraction grating formula. For this reason, light spots of the diffracted light emitted from the diffractive optical element may have a uniform distribution with respect to a spherical surface having the diffractive optical element at an approximate center thereof. However, when the diffracted light emitted from the diffractive optical element is projected on a plane or a flat surface, an interval (or spacing) of the light spots of the diffracted light becomes longer as the diffraction angle of the diffracted light becomes larger. When such a diffractive optical element is used in the coordinate measuring apparatus or the like, the diffraction angle of the diffracted light becomes large, and a detection sensitivity deteriorates in a region where the interval of the light spots is long, that is, in the region where a density of the light spots of the diffracted light is coarse, to thereby make an accurate three-dimensional measurement difficult. In the present specification, “approximately” refers to such a state that is observed by the naked eye or by an optical microscope including a stereomicroscope and the like.
On the other hand, when the diffracted light is projected onto the plane or the flat surface using the diffractive optical element, corner portions of a projection region of the diffracted light stretch when the diffracted light is distributed so that the projection region becomes an approximately rectangular shape, and the so-called pincushion distortion occurs.